Primate T-lymphotropic viruses (PTLVs) are diverse deltaretroviruses, composed of 3 distinct species (PTLV-1, -2, -3) which by conventional nomenclature are named ‘STLV’ (simian T-lymphotropic virus) when found in non-human primates (NHPs) and ‘HTLV’ (human T-lymphotropic virus) when found in humans, regardless of suspected zoonotic origin (Mahieux et al. 1998; Salemi et al. 1999; Slattery et al. 1999; Courgnaud et al. 2004). Like HIV, HTLV has the potential to cause disease and circulate globally in humans sexually, from mother-to-child, and by exposure to contaminated blood from transfusions and intravenous drug use. HTLV-1 causes adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraperesis (HAM/TSP) and other inflammatory diseases (Gessain & Mahieux 2000) and HTLV-2 has been associated with a neurologic disease similar to HAM/TSP (Araujo & Hall 2004). There has been no evidence to date of STLVs crossing into people occupationally exposed to NHPs in laboratories and primate centers, as has been documented with other primate retroviruses, including simian immunodeficiency virus (SIV) (Khabbaz et al. 1994), simian foamy virus (SFV) (Switzer et al. 2004, Heneine et al. 1998), and simian type D retrovirus (Lerche et al. 2001). Nevertheless, ongoing zoonotic transmission of STLV to widespread human populations naturally exposed to NHPs through hunting or butchering, similar to that recently reported for SFV in African hunters (Wolfe et al. 2004b), would be of particular public health significance due to the transmissible and pathogenic nature of this group of viruses among humans. HTLV outside of the PTLV-1 and PTLV-2 groups has not previously been documented (Busch et al. 2000; VanDamme et al. 1997; Salemi et al. 1999; Slattery et al. 1999).